Myocardial infarction remains a major cause of morbidity and mortality in the U.S. One important determinant of early and late mortality is the extent of left ventricular dysfunction. Although early reperfusion by either pharmacologic or mechanical means has been conclusively shown to reduce cardiac mortality, less striking beneficial effects on left ventricular function and infarct size reduction have been demonstrated. This finding, in conjunction with the observation that administration of the endogenous nucleoside Adenosine after reperfusion significantly enhances myocardial salvage in experimental models, supports the hypothesis that reperfusion per se may be detrimental to the previously ischemic but viable cardiac tissue ("REPERFUSION INJURY"). Reperfusion is associated with accelerated structural abnormalities in the vasculature resulting in progressive microcirculatory failure ("no- reflow" phenomenon). The etiology of this phenomenon remains unknown but may be produced by both hum oral and mechanical factors. ENDOTHELlN, a potent, long-acting vasoconstrictor peptide, has been shown to be significantly increased in the coronary sinus effluent in the experimental preparation of ischemia/reperfusion. Intravenous ADENOSINE, in a dose which enhances myocardial salvage, prevents this increase in the early reperfusion period. This proposal will attempt to define the role of ENDOTHELIN in the pathogenesis of reperfusion injury and the mechanisms whereby ADENOSINE modulates its release. Initial studies will determine the time course of the increase of ENDOTHELIN with various durations of ischemia. The role of ENDOTHELIN as an important mediator of reperfusion injury will be explored utilizing specific monoclonal ENDOTHELIN antibodies and a specific ENDOTHELIN receptor antagonist in the intact rabbit model. The mechanism whereby ADENOSINE attenuates the release and/or production of ENDOTHELIN will be determined utilizing both,in vivo and in vitro models. In situ hybridization techniques will be employed in both model systems to determine if ADENOSINE alters ENDOTHELIN gene production via activation of extracellular purinergic receptors. The role of activated neutrophils and platelets will be explored in an in vivo canine model of endothelial injury. The effect of ENDOTHELIN on neutrophil-endothelial interactions will be examined utilizing endothelial cell cultures. These studies will provide important information regarding the role of ENDOTHELIN in myocardial reperfusion injury and whether modulation of this peptide accounts for the beneficial effects of ADENOSINE on myocardial reperfusion injury further understanding of the mechanisms of action of ADENOSINE in protecting against reperfusion injury would allow for important advances in the treatment of evolving myocardial infarction.